The alkylation of aromatics with olefins to produce monoalkyl aromatics is a well developed art which is practiced commercially in large industrial units. One commercial application of this process is the alkylation of benzene with ethylene to produce ethylbenzene which is subsequently used to produce styrene. Another application is the alkylation of benzene with propylene to form cumene (isopropylbenzene) which is subsequently used in the production of phenol and acetone. Those skilled in the art are therefore familiar with the general design and operation of such alkylation processes.
The performances of alkylation processes for producing monoalkyl aromatics are influenced by the stability and activity of the solid catalyst at the operating conditions of the process. For example, as the molar ratio of aromatic per olefin increases, currently available catalysts typically exhibit an improved selectivity to the monoalkyl aromatic. But even at a high molar ratio of aromatic per olefin, several polyalkyl aromatic by-products such as dialkyl aromatics and trialkyl aromatics accompany monoalkyl aromatic production.
Although the formation of dialkyl and trialkyl aromatics might, at first glance, be viewed as by-products that represent a reduction in the efficient use of the olefin, in fact each can be readily transalkylated with the aromatic using a transalkylation catalyst to produce the monoalkyl aromatic. So-called combination processes combine an alkylation zone with a transalkylation zone in order to maximize monoalkyl aromatic production.
One disadvantage of combination processes is that separate reaction zones for alkylation and for transalkylation duplicate costly equipment. Each reaction zone requires what amounts to its own reaction train, including separate and independent reaction vessels, heaters, heat exchangers, piping, and instrumentation.
Another disadvantage of combination processes is the great expense associated with recovering and recycling unreacted aromatic from the effluents of the alkylation and transalkylation reaction zones. Alkylation reaction zones generally operate at a molar ratio of aromatic per alkylation agent that is at least 1:1 in order to ensure a high selectivity toward the monoalkyl aromatic. Transalkylation reaction zones generally operate at a molar ratio of aromatic per dialkyl aromatic that is much greater than the stoichiometric ratio of 1:1 in order to ensure a high conversion of the dialkyl aromatic to the monoalkyl aromatic. Consequently, the alkylation and transalkylation reaction zone effluents contain a significant quantity of unreacted aromatic, which must be removed in order to obtain the monoalkyl aromatic product and which must be recycled in order to ensure the efficient use of the aromatic.
Prior art combination processes lessen the great expense incurred in removing and recycling the unreacted aromatic contained in the alkylation and transalkylation reaction zone effluents by routing each alkylation and transalkylation effluent stream to a single, common product recovery facility, in which the very same distillation columns remove unreacted aromatic from both effluent streams and recycle unreacted aromatic to both reaction zones. Incidentally, a no less important function of these distillation columns in the prior art is the removal of polyalkyl aromatics other than dialkyl and trialkyl aromatics and of other heavy alkylation and transalkylation by-products such as diphenylalkanes, which are collectively referred to herein as heavies. Although sharing common product equipment in this manner reduces the capital expense of a combination process, the energy requirements for vaporizing and condensing the aromatic from the effluent streams remains undiminished.
Thus the high utilities expenses of combination processes as well as the costly duplication of reaction zones has given impetus to research with a goal of minimizing energy requirements and of integrating the alkylation and transalkylation zones even further.